components/LiteCharms.Features.MidrandBooks/Payments/PayfastService.cs

using LiteCharms.Features.Abstractions;
using LiteCharms.Features.Api.Configuration;
using LiteCharms.Features.Hasher;
using LiteCharms.Features.MidrandBooks.Payments.Models;
using LiteCharms.Features.MidrandBooks.Postgres;

namespace LiteCharms.Features.MidrandBooks.Payments;

public sealed partial class PayfastService(IDbContextFactory<MidrandBooksDbContext> contextFactory, 
    IOptions<PayfastSettings> payfastOptions, ILogger<PayfastService> logger, IHttpClientFactory httpClientFactory) : IService
{
    [GeneratedRegex(@"%[0-9A-Fa-f]{2}", RegexOptions.None, matchTimeoutMilliseconds: 1000)]
    public static partial Regex PercentEncodingRegex { get; }

    public async ValueTask<Result<long>> WriteLedgerEntryAsync(CreateGatewayLedgerEntry request, CancellationToken cancellationToken = default)
    {
        try
        {
            await using var context = await contextFactory.CreateDbContextAsync(cancellationToken);

            if(!await context.Orders.AnyAsync(o => o.Id == request.OrderId, cancellationToken))
                return Result.Fail<long>("Referenced order ID does not exist in database.");

            if(!await context.Payments.AnyAsync(p => p.Id == request.PaymentId, cancellationToken))
                return Result.Fail<long>("Referenced payment ID does not exist in database.");

            var entry = context.GatewayLedger.Add(new Entities.PaymentGatewayLedger
            {
                CustomerEmail = request.CustomerEmail,
                OrderId = request.OrderId,
                PaymentId = request.PaymentId,
                MerchantPaymentId = request.MerchantPaymentId,
                PayfastPaymentId = request.PayfastPaymentId,
                PaymentStatus = request.PaymentStatus,
                AmountGross = request.AmountGross,
                AmountFee = request.AmountFee,
                AmountNet = request.AmountNet,
                CreatedAt = DateTime.UtcNow,                
            });

            return await context.SaveChangesAsync(cancellationToken) > 0
                ? Result.Ok(entry.Entity.Id)
                : Result.Fail<long>("Failed to save Payfast ledger entry to database.");
        }
        catch (Exception ex)
        {
            return Result.Fail<long>(new Error("Failed to write Payfast ledger entry to database.").CausedBy(ex));
        }
    }

    public async ValueTask<Result<bool>> ValidateReferrerIpAsync(string remoteIpAddress, bool allowLoopback = false, CancellationToken cancellationToken = default)
    {
        if(payfastOptions.Value?.ValidHosts?.Length == 0)
            return Result.Fail<bool>("Valid payfast hosts not configured.");

        if (string.IsNullOrWhiteSpace(remoteIpAddress)) 
            return Result.Fail<bool>("Remote IP address is null or whitespace.");

        try
        {
            var validIps = new HashSet<IPAddress>();

            foreach (var host in payfastOptions.Value!.ValidHosts!)
            {
                try
                {
                    var addresses = await Dns.GetHostAddressesAsync(host, cancellationToken);

                    foreach (var addr in addresses) validIps.Add(addr);
                }
                catch (SocketException ex)
                {
                    logger.LogWarning(ex, "DNS warning: Failed to resolve Payfast node '{Host}'. It may be decommissioned or unreachable.", host);
                }
            }

            if (IPAddress.TryParse(remoteIpAddress, out var incomingIp))
            {
                if (allowLoopback && IPAddress.IsLoopback(incomingIp))
                {
                    logger.LogInformation("Local development loopback IP '{RemoteIp}' allowed bypassing DNS verification.", remoteIpAddress);
                    return Result.Ok(true);
                }

                bool isValid = validIps.Contains(incomingIp);

                if (!isValid)
                    logger.LogWarning("SECURITY ALERT: Webhook IP '{RemoteIp}' originated from an unlisted host schema.", remoteIpAddress);

                return Result.Ok(isValid);
            }

            return Result.Fail<bool>("Invalid remote IP address format.");
        }
        catch (Exception ex)
        {
            return Result.Fail<bool>(new Error("DNS Verification error while scanning Payfast IP nodes.").CausedBy(ex));
        }
    }

    public Result<bool> ValidatePaymentAmount(decimal expectedTotal, string? amountGrossString)
    {
        if (!decimal.TryParse(amountGrossString, CultureInfo.InvariantCulture, out decimal grossAmount))
            return Result.Fail<bool>("Failed to parse payment amount.");

        decimal delta = Math.Abs(expectedTotal - grossAmount);

        bool isAmountValid = delta <= 0.01m;

        if (!isAmountValid)
            logger.LogError("FINANCIAL DRIFT EXCEPTION: Expected order total R{Expected} but gateway cleared R{Cleared}.", expectedTotal, grossAmount);

        return Result.Ok(isAmountValid);
    }

    public async ValueTask<Result<bool>> ValidateServerConfirmationAsync(string rawQueryParamString, bool isSandbox, CancellationToken ct)
    {
        try
        {
            string host = isSandbox ? "sandbox.payfast.co.za" : "www.payfast.co.za";
            string targetUrl = $"https://{host}/eng/query/validate";

            using var content = new StringContent(rawQueryParamString, Encoding.UTF8, "application/x-www-form-urlencoded");

            var httpClient = httpClientFactory.CreateClient();

            var response = await httpClient.PostAsync(targetUrl, content, ct);

            if (!response.IsSuccessStatusCode) return Result.Fail<bool>("Failed to validate server confirmation.");

            string responseText = await response.Content.ReadAsStringAsync(ct);

            bool isValidated = string.Equals(responseText.Trim(), "VALID", StringComparison.OrdinalIgnoreCase);

            if (!isValidated)
                logger.LogWarning("SECURITY WARNING: Payfast back-channel returned validation response: '{Response}'", responseText);

            return Result.Ok(isValidated);
        }
        catch (Exception ex)
        {
            return Result.Fail<bool>(new Error("Failed to complete back-channel cURL verification handshakes with Payfast remote endpoints.").CausedBy(ex));
        }
    }

    public static Result<string> GenerateSignature(IDictionary<string, string?> data, string? passPhrase = null)
    {
        var pfOutput = new StringBuilder();

        var mandatorySequence = GetPayfastMandatoryFieldSequence();

        foreach (string key in mandatorySequence)
        {
            if (data.TryGetValue(key, out string? rawValue) && !string.IsNullOrEmpty(rawValue))
            {
                string encodedVal = HttpUtility.UrlEncode(rawValue.Trim());
                string val = PercentEncodingRegex.Replace(encodedVal, m => m.Value.ToUpperInvariant());

                pfOutput.Append($"{key}={val}&");
            }
        }

        var getString = pfOutput.Length > 0
            ? pfOutput.ToString()[..^1]
            : string.Empty;

        if (!string.IsNullOrWhiteSpace(passPhrase))
        {
            string encodedPassphrase = HttpUtility.UrlEncode(passPhrase.Trim());
            string safePassphrase = PercentEncodingRegex.Replace(encodedPassphrase, m => m.Value.ToUpperInvariant());

            getString += $"&passphrase={safePassphrase}";
        }

        return HashService.ToMd5Hash(getString);
    }

    private static string[] GetPayfastMandatoryFieldSequence() =>
        [
            "merchant_id",
            "merchant_key",
            "return_url",
            "cancel_url",
            "notify_url",
            "name_first",
            "name_last",
            "email_address",
            "cell_number",
            "m_payment_id",
            "amount",
            "item_name",
            "item_description",
            "custom_int1",
            "custom_int2",
            "custom_int3",
            "custom_int4",
            "custom_int5",
            "custom_str1",
            "custom_str2",
            "custom_str3",
            "custom_str4",
            "custom_str5",
            "email_confirmation",
            "confirmation_address",
            "payment_method",
            "subscription_type",
            "billing_date",
            "recurring_amount",
            "frequency",
            "cycles"
        ];
}